1. Field of the Invention
The present invention relates to the recovery of metal values from cermet material, especially cermet material of which inert cermet positive and negative electrodes (anodes) are comprised. Such inert cermet anodes include inert or non-consumable electrodes used in the production of aluminum by electrolytic reduction of alumina dissolved in a molten salt bath. In particular this invention pertains to a composition comprising fired and/or unfired cermet in a form suitable for the recovery of metal values therefrom in a smelter, especially a nickel or copper smelter, and to a smelting process which uses this composition as feedstock by itself or with ore and/or ore concentrate.
2. Background Information
Aluminum has been produced using the well known Hall-Heroult cell since Charles Martin Hall""s invention for a process of reducing aluminum from its fluoride salts by electrolysis which is the subject of U.S. Pat. No. 400,664 issued on Apr. 2, 1889. In this electrolytic reduction process aluminum oxide (e.g., alumina or Al2O3) is dissolved in a bath of molten salt. The aluminum content of the alumina is reduced to metallic or elemental aluminum by an electrolytic process in which the aluminum of the aluminum oxide is reduced at the anode whereby metallic or elemental aluminum is produced. For many years carbon anodes were used in this process. The carbon anodes are consumed in the process as the carbon reacts with the alumina to produce elemental aluminum and carbon dioxide during electrolysis.
Recently inert anodes have been introduced for use in electrolytic production of aluminum. These inert anodes have the advantage of not being consumed during the reduction of the aluminum. Consequently these inert anodes are also referred to as non-consumable anodes or as dimensionally-stable anodes.
The inert or non-consumable anodes must be able to withstand the harsh conditions in which they are used (i.e., a molten salt bath which contains dissolved alumina). Furthermore, since these anodes are not consumed during the process for making aluminum, they must withstand these extremely harsh conditions for a considerable length of time. In particular the inert anode material must satisfy a number of difficult conditions. For example, the material must not react with or dissolve to any significant extent in the cryolite electrolyte which is typically used in the Hall-Heroult process. The anode material must not react with oxygen or corrode in an oxygen-conitaining atmosphere. This material should be thermally stable at temperatures of about 1000xc2x0 C. and should have good mechanical strength. The anode material must have electrical conductivity greater than 120 ohmxe2x88x921cmxe2x88x921at the smelting cell operating temperature about 950xc2x0-970xc2x0 C. In addition, aluminum produced with the inert anodes should not be contaminated with constituents of the anode material to any appreciable extent.
Inert anodes made from cermet material have been found to satisfy the above-mentioned conditions, thus making them particularly suitable in the Hall-Heroult process.
Cermets are composite materials which have a ceramic phase and a metallic phase. They have the unique property which combines the desirable features of ceramics and metals including chemical inertness and electrical conductivity. Examples of inert anodes made from a cermet are described in U.S. Pat. Nos. 5,865,980 and 6,030,518, the specifications of which are incorporated herein by reference.
Because of the extraordinarily harsh operating environment of the cell, eventually these inert anodes made from cermet need to be replaced. Replacing the used anodes with new ones has created a disposal problem with a loss of the valuable metal components thereof. Since a typical inert anode contains combinations of metals that may include nickel, silver, copper and iron, disposal of these anodes represents a significant loss to the aluminum industry if these metals are not recovered and either sold or recycled. An inert anode described in U.S. Pat. No. 5,865,980 contains 14 wt. % copper, 7% silver, 40 wt. % nickel oxide, 38 wt. % iron and traces of other metals. Thus disposing of these anodes without recovering the metal values therefrom will be wasteful and economically disadvantageous.
Oxides of tin are also found in some inert anode materials (JOM Light Metals 1996, xe2x80x9cInert Anodes for the Primary Aluminum Industryxe2x80x9d by Rudolf Pawiek, and JOM Light Metals, May 2001, xe2x80x9cCell Operations and Metal Purity Challenges for the use of Inert Anodesxe2x80x9d by Thoustad and Olsenxe2x80x9d).
The composition and characteristics of inert anodes which are used in the aluminum producing industry are discussed in an article in JOM Light Metal Age, February 2001 by Joseph Benedyk. It is noted in this article that the cermet consists of a ceramic phase and a metallic phase wherein the ceramic phase may be a matrix of nickel ferrite having a dispersion therein of a metallic phase which, for example, may be a nonferrous alloy such as copper or silver.
In addition to the used cermet anodes, there are also waste cermet anodes due to breakage, cermet ingredient materials and residues produced during the manufacturing process of the inert anodes and inert anodes that have failed to meet quality control standards. The same problems noted above with respect to the used anodes, also applies to the waste cermet associated with the above-identified materials. Thus the above-noted problems apply to used and unused inert anode and the manufacturing residues.
Although it is highly desirable to recover the valuable metals from the above-noted anode materials, no one has ever suggested any economically feasible method for their recovery, despite the need in the industry for solving this problem. This is believed to result from the fact that the inert characteristic and other characteristics which make these anodes resist the harsh conditions within an electrolytic aluminum reduction cell, make the recovery of metal values from these anodes extremely difficult and challenging. Prior to this invention no economically viable methods were known for recovering the metal values from these inert anodes. It has now been discovered by the inventors that the metal values from these inert anodes and anode materials may be economically recovered by smelting, especially in a conventional nickel or copper smelter, by converting the cermet of the inert anodes into a composition which can be smelted in the smelter.
Rath in U.S. Pat. No. 4,119,454 discloses a method for recovering ferrous metal values from steel scrap. The process employs a smelting step in which the steel scrap is fed into a smelter which produces a slag layer on top and a molten layer underneath the slag layer. The process provides for the separate recovery of the slag and metal layers. Rath does not disclose or suggest the recovery of metal values from cermet material in general nor specifically from inert anodes which comprise cermet. Furthermore, Rath does not disclose or suggest a cermet composition in a form which can be readily smelted in a conventional smelter. In addition, Rath is not in any way concerned with solving the technical problems associated with recovering metal values from an extremely inert composition which is designed to resist the harsh conditions utilized in aluminum smelting.
Kapanen et al. in U.S. Pat. No. 4,029,494 disclose a process and apparatus for recovering noble metal values from anode slime produced in an electrolytic copper process. The anode slime containing the recoverable noble metals is subjected to a smelting procedure. Kapanen et al. do not disclose or suggest using their procedure to recover metal values from anodes which comprise cermet. In addition, Kapanen et al. are not in any way with solving the technical problems noted above with respect to recovery of metal values from inert cermet material which is designed to withstand the harsh conditions in aluminum smelting.
Sancinelli in U.S. Pat. No. 5,186,740 discloses the pretreatment of scrap prior to a smelting procedure in which metal values are recovered from the scrap. The pretreatment includes reducing the size of the scrap before it is introduced into a smelter and separating components such as organic materials from the scrap prior to the smelting procedure. Sancinelli does not disclose or suggest any process for recovering metal values from inert anodes which comprise cermet. Furthermore, since Sancinelli is not concerned with the recovery of metal values from cermet, he does not address any of the unique problems associated with recovery of metal values from inert cermet which is specifically designed to withstand the harsh conditions in aluminum smelting.
Elmore et al. in U.S. Pat. No. 4,118,219 disclose a process in which components of lead-acid batteries are subjected to a smelting procedure for the recovery of metal values therefrom. In this procedure a solid metal fraction is isolated and sent to a refinery where it is dried, melted and/or smelted and refined to produce lead alloys which can be re-used in new batteries. Elmore et al. disclose the use of flux in the smelting procedure and further disclose the use of a carbon additive as a reductant in the smelting procedure. However, Elmore et al. do not disclose or suggest the recovery of metal value from inert anodes which comprise cermet and they are not in any way concerned with overcoming the above-noted technical problems associated with recovery of metal values from such an inert material like cermet.
Ogawa et al. in U.S. Pat. No. 4,274,785 disclose the introduction of anode scrap into a converter furnace. The anode scrap functions as a cooling material when it is introduced into the furnace. Ogawa et al. do not disclose or suggest the recovery of metal values from inert anodes which comprise cermet and they do not address any of the above-noted technical problems associated with recovery of metal values from such an inert material.
U.S. Pat. Nos. 3,393,876 and 3,689,253 are of additional interest since they disclose a smelting procedure for the recovery of lead from batteries.
None of the above-noted references address the unique problems associated with the recovery of metal values from cermet material which is designed to withstand the harsh conditions within an aluminum smelter and none of these references disclose or suggest the formation of a cermet material in a form from which metal values can be recovered under metal recovery conditions in a smelter.
It is possible to separate elemental metal from other components, but such separation techniques are not suitable for the recovery of the metal values from cermet and furthermore these techniques do not recover metal values from metal compounds found in the cermet.
It is an objective of the present invention to provide a composition which comprises cermet material, especially used and unused, in a form which is suitable for smelting so that metal values from the cermet may be recovered in a smelting procedure.
It is also an objective of the present invention to recover metal values from a composition which comprises cermet material, using a smelter, especially a nickel or copper smelter.
These and other objectives are achieved by first obtaining the cermet material from which the metal values are to be recovered. Suitable sources of the cermet include but are not limited to used and unused inert anodes which contain cermet and cermet used in and/or from the manufacturing of the inert anodes. Cermet used in the manufacturing of the inert anodes includes inert anode manufacturing residue, and inert unused anode from the manufacturing facility. Other cermet containing materials or articles may be used as the source for cermet.
Any of the above-mentioned sources of cermet, including any combination of these sources (henceforth referred to herein as inert anode material), is first qualified and characterized using physical/analytical characterization to determine the recyclability of the inert anode material.
Physical characterization is carried out to determine material friability and to determine whether the material is sufficiently free of debris and safe to handle for recycling. Analytical characterization is carried out to determine the mineral and metal constituents and their content and to determine if the inert anode material is suitable to produce a concentrate material feedstock for a smelter based on specific smelter concentrate feedstock specifications. Analytical characterization is also conducted to determine the recoverable metal value, mineral content, impurity levels and levels of constituents which may be deleterious to the smelting process which will be used to recover the desired metal values.
Next the inert anode material is beneficiated to produce the concentrate of this invention using beneficiation techniques which are well known to those skilled in the art of ore mining and metallurgy technology. Such beneficiation processes include any conventional sorting and size reduction to achieve the desired material handling flow and particle size characteristics conducive to the smelting procedure. These characteristics are specifically related to the smelting process parameters and the selected type of metal concentrate product technically acceptable for the smelting process. If the source of cermet includes non-cermet components, these components are desirably separated from the cermet as part of the beneficiation process. For example, in the case where inert anodes having nickel or nickel-chrome rods (JOM Light Metal Age 2001, xe2x80x9cInert Anodes for the Hall-Heroult Cell: The Ultimate Material Challengexe2x80x9d by Joseph C. Benedyk, May 2001) are used as a source for cermet, non-cermet components such as the rods or other metal components are desirably removed as part of the process of beneficiation.
In some instances the source of the cermet will not contain any non-cermet component. In those instances the cermet is beneficiated solely by comminution to produce the concentrate of this invention. Since there are no non-cermet components to be removed, this beneficiated cermet is the same as the beneficiated inert anode material from which non-cermet components have been removed as part of the beneficiation process.
Additives (e.g., metallurgical fluxing reagents, other beneficial ingredient additives including other metal bearing materials, ores or ore concentrates) which are needed or useful to achieve desired metallurgical quality specifications for the resulting concentrate produced from the beneficiated inert anode material prior to the introduction into the smelter, are desirably added prior to the subsequent smelting process. These ingredient additives are advantageously mixed with the beneficiated inert anode material (i.e., the concentrate) to formulate a concentrate composition containing additives that can be fed into the smelter for the recovery of the metal values therefrom. Binders and/or dust suppressants are desirably added to the beneficiated inert anode based concentrate so that it may be agglomerated and/or pelletized to thereby form a suitable concentrate of this invention from which the metal values can be recovered in the smelting procedure.
The term xe2x80x9cconcentratexe2x80x9d as used herein refers to material which has a sufficiently high level of metal (i.e., concentration) to be recovered in a smelting process which uses a primary smelter regardless of whether any concentration steps have been taken to make the concentrate. Typically ores require extensive removal of earthy and valueless constituents during beneficiation of the ore to obtain the desired concentration of the metal to be recovered. The beneficiation of inert anode material does not require extensive concentration steps.
The concentrate which includes the additives and the beneficiated inert anode material or other beneficiated cermet constitutes one aspect of this invention. This concentrate with the additives such as fluxing reagent contained therein, may be sent to a conventional smelter, with the additives such as fluxing reagent contained therein, for the recovery of the metal values contained therein.
Alternatively, the fluxing additives may be added at the smelter along in a process called bedding. In the bedding process the beneficiated inert anode material (i.e., the concentrate of this invention), is formulated with desired proportions of required fluxes such that when the bedded material is removed to the smelter, the concentrate is removed with the appropriate quantity of flux.
The concentrate material, which preferably includes the additives and is preferably in an agglomerated form, may be roasted under oxidizing conditions prior to the introduction of the concentrate into the smelter to begin the impurity removal process and to oxidize certain constituent compounds.
The above-noted concentrate represents one aspect of this invention which pertains to a composition which consists essentially of isolated cermet material in a form which is suitable for conventional nickel and/or copper smelting so that metal values from the cermet may be recovered in a smelting procedure.
This invention also pertains to the use of the aforementioned concentrate in a smelter to recover the metal values from the inert anode material. Thus in another aspect this invention pertains to a smelting procedure wherein the feed to the smelter comprises the aforementioned concentrate which contains the inert anode material in a form suitable for smelting.
The smelter used in the process of this invention is a primary smelter, which is one that has been designed to extract nickel or copper along with other associated metal values from ore. The term xe2x80x9cprimaryxe2x80x9d, principally denotes that the metals extracted are from ore (i.e., primary smelter) and not from a source which is typically metal scraps (secondary smelter). Primary smelters are preferred for use in the smelting process of this invention because they have the ability to efficiently and economically extract and recover the valuable metals from the concentrate of this invention. In addition, by using the metallurgical process of a primary smelter, the concentrate of this invention is advantageously combined with ore concentrates in the smelting process thereby obtaining the efficiencies and favorable economics associated with primary smelting. The primary smelting process has the following characteristics which distinguishes the process from secondary smelting.
Importantly, the principal function of the primary smelting process is to extract metals of value from concentrates. Chemical reduction during the molten phase of this process accomplishes this wherein fusion of the ore and concentrate impurities report to the slag which is comprised of fluxing reagents that help control both the viscosity of the total molten mass and density of the resulting fusible slag. The resulting lower density of the molten slag gravimetrically separates from the mass and floats to the surface where it is then removed for either disposal or reprocessing by its reintroduction to the smelter to recover any remaining values. Slags that are reprocessed are called reverts.
Secondary smelting, although flux reagent can be used, focuses on remelting the metals of value which originate from metallic scrap and not ore and concentrate. Re-melting facilitates forming and shaping of the metal for fabrication rather than extracting the metal from earthy components or other impurities.
The inert anode material used in the present invention contains the following metal or metals which can be recovered in abundance with this invention:
Although different types of smelting processes and corresponding apparatus may be used in accordance with this invention, two of the more common smelting processes include the so-called continuous and flash techniques designed for sulfidic copper and/or nickel concentrates. Thus, copper and nickel smelters are the preferred for use in this invention. Copper and nickel smelting processes and their corresponding apparatus are especially useful in cases where the inert anode material contains precious metals such as silver and gold, or other platinum group metals.
The term xe2x80x9csmeltingxe2x80x9d is well known to those skilled in the art and is a generic description for the chemical reduction of metal from its ore or concentrate by a process usually involving fusion, so that the earthy and other impurities, separating as lighter and more fusible slags, can readily be removed from the reduced metal. Generally smelting is understood by those skilled in the art as a process which is distinct from roasting, sintering, fire refining, and other pyrometallurgical operations. However in the newer technologies of flash or continuous smelting, some of these steps are combined.
The two most important steps of the primary smelting process for copper and/or nickel are reduction smelting which produces molten matte and molten slag, and matte smelting which produces molten blister and molten slag. Smelting to produce matte may be conducted in a reverberatory furnace, an electric furnace, a continuous furnace, or a blast furnace whereas, blister, the next stage, is usually performed in furnace called a converter, but there are exceptions in each case.
Typically the concentrate composition of this invention is used in the reduction smelting processes which produce molten matte and molten slag.
In the process of reduction smelting, the precious and platinum group metals together with the nonferrous metals such as cobalt, nickel and copper report to the matte, rather than to the slag, during the smelting process whereby they are accumulated and after converting remain in the blister. It is a relatively standard procedure to then recover the individual metal values by standard metallurgical processes such as electrowinning. Thus metal refining of the blister produces recovered precious metals, platinum group metals, and nonferrous metals such as nickel, cobalt and copper originally contained in the cermet.
In some instances it maybe desirable to recover nonferrous alloys directly from the blister without an intermediate metal refining procedure. This, however would normally be in cases where precious metals and/or platinum group metals are not present in the blister and the metal content of the blister is such, that as an alloy, it can be directly shaped and used for fabrication.
The slag produced by the smelting process may be recovered and used in the construction industry according to known methods. For example, the slag may be used as road aggregate, railroad ballast, blasting media, or as an ingredient in Portland cement. Alternatively, it may be disposed according to known disposal procedures for the safe disposal of slag.
It is also normal in smelting process to recycle the slag or a portion thereof as a reverts back to the smelter for the recovery of remaining metal values therefrom.
The present invention is advantageous because the beneficiated inert anode material may be returned to the inert anode manufacturing facility and incorporated into newly fabricated inert anodes. When using the beneficiated inert anode material in manufacturing new inert anodes, the beneficiated material must be qualified to ensure that it is within the specifications for the ingredients used in the inert anode manufacturing process. Alternatively the beneficiated inert anode material may be sent to the inert manufacturing facility for incorporation into newly fabricated inert anodes without prior qualification (physical and/or analytical), in which case the operator of the inert manufacturing facility will test the material to see if it meets manufacturing quality control standards. Economically the product life cycle may be enhanced by reuse in manufacturing, however, only a selected fraction of the beneficiated material may qualify for recycling to the manufacturing procedure, whereas with the smelting method of this invention, the total quantity of beneficiated material may be processed for the economic recovery of metal values therefrom. Moreover, the concentrate product of this invention, being in a form which can be smelted for the recovery of metal values therefrom, is a valuable commodity which can be sold to primary smelters for use as a smelting metal source feedstock by itself or in combination with ore-based smelting feedstock. This aspect of the invention is particularly advantageous because it creates a valuable marketable material commodity which would otherwise have to be disposed at considerable expense and loss of valuable metal content.